This invention relates generally to electrical apparatus, and more particularly to printed circuit boards and the method of manufacturing printed wiring or circuit boards.
State of the art printed circuit and wiring boards are requiring smaller and smaller circuitry. The present subtractive and semi-additive processes, though they can in many cases meet the needs of the industry today, are rapidly being pushed to the limit of their capability to provide the fine circuitry features required. Tighter controls are needed to even attempt fine line boards, and yields drop off dramatically as the lines and spaces get smaller.
In conventional printed wiring technology, circuitry is fabricated in layer pairs, with a layer of circuitry being disposed on either side of a sheet of dielectric material. The layer pairs are laminated together and drilled and plated to make the completed multilayer structure. The conventional method of etching back copper clad dielectric material to produce circuitry has proved to be inadequate when dealing with extremely fine lines. A major problem with this prior art method is the undercutting that accompanies the etching of the copper cladding. Etching the sides of the circuitry decreases the line width in such a way that the circuitry does not always have straight sides. The etching is also not completely uniform, so that some lines will have more of their sides etched away than other lines. The tolerances that are inherent with the etching process are not good enough to meet the very large scale integrated circuit requirements.
Another problem in the prior art fabrication of printed circuit and wiring boards concerns the method of making interconnections between the various layers of a multilayer board. This prior method involves drilling holes through copper and board material, and then plating copper through the hole to make the electrical connection. Because of problems associated with attempting this copper plating process, a good percentage of failures of printed wiring boards are related to the plated through holes. Furthermore, in order to accommodate this method of interconnecting, the art work utilized in fabricating the boards is designed with pads at the hole locations. These pads must be several times larger than the circuitry in order to provide a big enough target for the drill. The density of the circuitry, therefore, is limited because of the large pads that must be worked around.
U.S. Pat. No. 4,606,787, issued Aug. 19, 1986 and entitled "Method and Apparatus for Manufacturing Multilayer Printed Circuit Boards", describes a method by which printed circuit boards are fabricated using an additive process for creating the conductive lines on the board. This method eliminates the etching difficulties, but still retains the hole plating procedure for providing interlayer circuit connections.
Another disadvantage of the prior art fabrication of printed wiring boards concerns the pads utilized for the attachments of the very large scale integrated (VLSI) circuit packages which are to be mounted on the boards. In particular, the solder deposition and lead separation of high density VLSI printed wire assemblies has been a difficult task. Solder masks have been investigated; however, many are difficult to process, are unable to withstand extreme environmental conditions, and are not able to be fabricated to the applicable size requirements.
One common method of surface mounting the VLSI component assemblies on the boards includes soldering the leads of the components to pads on the board. On boards with a large number of pads, it is necessary to find an alternate method to soldering the leads individually. If enough solder is located at each pad, the board can be heated using a vapor phase or comparable process and all the component leads can be soldered simultaneously. Though there exists several methods for placing the solder on the pads, there exists a problem with dense circuitry, for unless the solder is contained, it can flow between the pads and form solder bridges. These bridges then short circuit the board, rendering it useless.